Index ausserhalb des gültigen Bereichs

Private Function sheet_exist(ShName As String) As Boolean
Dim sh As Object
sheet_exist = False
For Each sh In ThisWorkbook.Sheets
If sh.Name = ShName Then
sheet_exist = True
Exit For
End If
Next
End Function

Private Sub Speichern()
Application.ScreenUpdating = False
Dim sh As Object
Dim lZ As Long
Dim rngFind As Range                                               Fehlermeldung
Set sh = Sheets(Format(Cells(11, 11), "MM.YYYY"))                <--------------
If sheet_exist(Format(Cells(11, 11), "MM.YYYY")) = False Then
Sheets.Add After:=Sheets(Worksheets.Count), Type:=xlWorksheet
Sheets(Worksheets.Count).Name = Format(Cells(11, 11), "MM.YYYY")
End If